Mounted UV Fused Silica Reflective ND Filters


  • UV to IR Spectral Range (200 - 1200 nm)
  • Metallic Nickel Coating on UV Fused Silica Substrate
  • Optical Densities from 0.1 to 4.0 Available

NDUV2R02A

Ø50 mm

NDUV520A

Ø1/2"

NDUV503A

Ø1/2"

NDUV02A

Ø25 mm

NUK01

Box of 10 Mounted
Ø25 mm UVFS Filters

Related Items


Please Wait
Neutral Density Filter
Selection Guide
Absorptive
Uncoated
(400 - 650 nm)
Mounted
Unmounted
Uncoated
(1000 - 2600 nm)
Mounted
Unmounted
AR Coated
(350 - 700 nm)
Mounted
Unmounted
AR Coated
(650 - 1050 nm)
Mounted
Unmounted
AR Coated
(1050 - 1700 nm)
Mounted
Unmounted
Variable
Reflective
UV Fused Silica
(200 - 1200 nm)
Mounted
Unmounted
N-BK7
(350 - 1100 nm)
Mounted
Unmounted
ZnSe
(2 - 16 µm)
Mounted
Unmounted
Wedged UVFS (200 - 1200 nm)
Wedged N-BK7 (350 - 1100 nm)
Wedged ZnSe (2 - 16 µm)
Variable
Neutral Density Filter Kits
IR Neutral Density Filters
Click to Enlarge

Transmission and Optical Density of Reflective UVFS ND Filters
Optic Cleaning Tutorial

Features

  • Optimized for Light Attenuation in the UV
  • Mounted Ø1/2", Ø25 mm, or Ø50 mm Filters
  • SM05-, SM1-, or SM2- Threaded Mounts
  • Optical Densities Available from 0.1 - 4.0
  • Optimized for 200 to 400 nm
  • Usable from 200 to 1200 nm
  • Ø25 mm Filter Kit Available

Thorlabs offers reflective neutral density (ND) filters made from UV fused silica substrates with nickel coatings deposited on one side, which provides a flat spectral response. These mounted metallic filters are usable in the 200 - 1200 nm spectral range and are available in Ø1/2", Ø25 mm, and Ø50 mm versions. The Ø1/2" mounted filters come in SM05 (0.535"-40)-threaded lens tubes, while the Ø25 mm and Ø50 mm versions are housed in SM1 (1.035"-40)- and SM2 (2.035"-40 )-threaded lens tubes respectively. Each filter is engraved with the part number, filter type (i.e., reflective), and optical density. For other mounting options and wavelength ranges, please see the Selection Guide table on the right.

The UV fused silica substrate used in these filters exhibits high transmission and virtually no laser-induced fluorescence, as measured at 193 nm, making it an ideal choice for applications from the UV to the near IR. While the spectral range's lower limit of 200 nm is limited by the absorption of the light by the substrate, UV fused silica provides good transmission up to 2.1 µm, and thus the upper limit of 1200 nm is dependent on the increased opacity of the nickel coating. The optical density (OD) for each filter is specified at the design wavelength of 300 nm to facilitate their use in the UV; some variation in the OD will occur over the usable range. For plots showing the typical performance of the filters from 200 to 2600 nm, click on info in the row corresponding to the desired filter in the tables below.

Unprotected metal coatings like this should only be cleaned by blown air, never touched, as contact may cause scratching to the unprotected surface. Although these are reflective ND filters, the nickel coating does absorb some of the incident light, which limits the use of these filters to low-power applications. Nickel is resistant to aging under normal conditions; however, it will oxidize at elevated temperatures. To prevent oxidation, Thorlabs recommends using these ND filters at temperatures below 100 °C. To achieve the best performance, light should be incident on the side with the nickel coating; this allows attenuation to occur before etalon effects occur within the filter's substrate. 

If desired, a mounted filter can be removed from its housing by unscrewing the retaining ring that secures the filter to the mount. Thorlabs offers a range of spanner wrenches that are an ideal match to these retaining rings. The round filters listed on this page are also sold unmounted. The equivalent unmounted filter is listed in the drawing for each mounted filter, accessible through the red docs icons () below.

Ten of the Ø25 mm mounted UVFS reflective ND filters are also offered in the NUK01 filter kit, sold below. This kit comes in the KT01 hard-plastic storage box with labeled foam inserts to organize the optics. The KT01 box is also sold separately below for storage of individually purchased mounted filters.

Optical Density and Transmission
Optical density (OD) indicates the attenuation factor provided by an optical filter, i.e. how much it reduces the optical power of an incident beam. OD is related to the transmission, T, by the equation

Optical Density Equation

where T is a value between 0 and 1. Choosing an ND filter with a higher optical density will translate to lower transmission and greater absorption of the incident light. For higher transmission and less absorption, a lower optical density would be appropriate. As an example, if a filter with an OD of 2 results in a transmission value of 0.01, this means the filter attenuates the beam to 1% of the incident power. Please note that the transmission data for our neutral density filters is provided in percent (%).

Please note that these products are not designed for use as laser safety equipment. For lab safety, Thorlabs offers an extensive line of safety and blackout products, including beam blocks, that significantly reduce exposure to stray light.

Filter Size Ø1/2" Ø25 mm Ø50 mm
Substrate Material UV Fused Silicaa
Front Surface Coating Nickel
Wavelength Range Optimized for 200 - 400 nm
Usable from 200 - 1200 nm
Optical Density Toleranceb ±5%
Optic Diameter 1/2" 25.0 mm 50.0 mm
Optic Diameter Tolerance +0.00 / -0.20 mm
Clear Aperture >Ø11.0 mm >Ø22.5 mm >Ø45.0 mm
Housing Thread SM05 (0.535"-40) SM1 (1.035"-40) SM2 (2.035"-40)
Housing Diameter 0.70" (17.8 mm)c 1.20" (30.5 mm) 2.20" (55.9 mm)
Filter Thickness 1 mm  2 mm 
Thickness Tolerance ±0.10 mm
Surface Flatness @ 633 nm <2λ <2λ per Ø25.0 mm
Parallelism <20 arcmin <30 arcsec <30 arcsec
Surface Quality 40-20 Scratch-Dig
Operating Temperatured <100 °C
  • Click Link for Detailed Specifications on the Substrate
  • The optical density tolerance is specified at 300 nm.
  • NDUV530A and NDUV540A have a 0.7" (17.78 mm) housing diameter.
  • The nickel coating will oxidize at temperatures greater than 100 °C.

Optical Density Damage Threshold
0.3 0.025 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.772 mm)
1.0 0.05 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.772 mm)
2.0 0.075 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.772 mm)
Damage Threshold Specifications
Optical Density Damage Threshold
0.3 0.025 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.772 mm)
1.0 0.05 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.772 mm)
2.0 0.075 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.772 mm)

Damage Threshold Data for Thorlabs' UV Reflective ND Filters

The specifications to the right are measured data for Thorlabs' UV reflective ND filters. Damage threshold specifications are constant for a given optical density, regardless of the size of the filter.

 

Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

LIDT metallic mirror
The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm2 (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
LIDT BB1-E02
Example Test Data
Fluence # of Tested Locations Locations with Damage Locations Without Damage
1.50 J/cm2 10 0 10
1.75 J/cm2 10 0 10
2.00 J/cm2 10 0 10
2.25 J/cm2 10 1 9
3.00 J/cm2 10 1 9
5.00 J/cm2 10 9 1

According to the test, the damage threshold of the mirror was 2.00 J/cm2 (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

Linear Power Density Scaling

LIDT in linear power density vs. pulse length and spot size. For long pulses to CW, linear power density becomes a constant with spot size. This graph was obtained from [1].

Intensity Distribution

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

  1. Wavelength of your laser
  2. Beam diameter of your beam (1/e2)
  3. Approximate intensity profile of your beam (e.g., Gaussian)
  4. Linear power density of your beam (total power divided by 1/e2 beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

CW Wavelength Scaling

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10-7 s and 10-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

Pulse Duration t < 10-9 s 10-9 < t < 10-7 s 10-7 < t < 10-4 s t > 10-4 s
Damage Mechanism Avalanche Ionization Dielectric Breakdown Dielectric Breakdown or Thermal Thermal
Relevant Damage Specification No Comparison (See Above) Pulsed Pulsed and CW CW

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

Energy Density Scaling

LIDT in energy density vs. pulse length and spot size. For short pulses, energy density becomes a constant with spot size. This graph was obtained from [1].

  1. Wavelength of your laser
  2. Energy density of your beam (total energy divided by 1/e2 area)
  3. Pulse length of your laser
  4. Pulse repetition frequency (prf) of your laser
  5. Beam diameter of your laser (1/e2 )
  6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm2. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e2 beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm2 at 1064 nm scales to 0.7 J/cm2 at 532 nm):

Pulse Wavelength Scaling

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Pulse Length Scaling

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10-9 s and 10-7 s. For pulses between 10-7 s and 10-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.


[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).

In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.

Intensity Distribution
A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile.

CW Laser Example
Suppose that a CW laser system at 1319 nm produces a 0.5 W Gaussian beam that has a 1/e2 diameter of 10 mm. A naive calculation of the average linear power density of this beam would yield a value of 0.5 W/cm, given by the total power divided by the beam diameter:

CW Wavelength Scaling

However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm.

An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value:

CW Wavelength Scaling

The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application.

Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations
Suppose that a pulsed Nd:YAG laser system is frequency tripled to produce a 10 Hz output, consisting of 2 ns output pulses at 355 nm, each with 1 J of energy, in a Gaussian beam with a 1.9 cm beam diameter (1/e2). The average energy density of each pulse is found by dividing the pulse energy by the beam area:

Pulse Energy Density

As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm2.

The energy density of the beam can be compared to the LIDT values of 1 J/cm2 and 3.5 J/cm2 for a BB1-E01 broadband dielectric mirror and an NB1-K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration:

Pulse Length Scaling

This adjustment factor results in LIDT values of 0.45 J/cm2 for the BB1-E01 broadband mirror and 1.6 J/cm2 for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm2 maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system.

Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths
Suppose that a pulsed laser system emits 10 ns pulses at 2.5 Hz, each with 100 mJ of energy at 1064 nm in a 16 mm diameter beam (1/e2) that must be attenuated with a neutral density filter. For a Gaussian output, these specifications result in a maximum energy density of 0.1 J/cm2. The damage threshold of an NDUV10A Ø25 mm, OD 1.0, reflective neutral density filter is 0.05 J/cm2 for 10 ns pulses at 355 nm, while the damage threshold of the similar NE10A absorptive filter is 10 J/cm2 for 10 ns pulses at 532 nm. As described on the previous tab, the LIDT value of an optic scales with the square root of the wavelength in the nanosecond pulse regime:

Pulse Wavelength Scaling

This scaling gives adjusted LIDT values of 0.08 J/cm2 for the reflective filter and 14 J/cm2 for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage.

Pulsed Microsecond Laser Example
Consider a laser system that produces 1 µs pulses, each containing 150 µJ of energy at a repetition rate of 50 kHz, resulting in a relatively high duty cycle of 5%. This system falls somewhere between the regimes of CW and pulsed laser induced damage, and could potentially damage an optic by mechanisms associated with either regime. As a result, both CW and pulsed LIDT values must be compared to the properties of the laser system to ensure safe operation.

If this relatively long-pulse laser emits a Gaussian 12.7 mm diameter beam (1/e2) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10-4 J/cm2 per pulse. This can be compared to the LIDT values for a WPQ10E-980 polymer zero-order quarter-wave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm2 for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm2 for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a high-power CW beam.

Threading Specifications

The following is a general overview of screw threading. For more details regarding specifications and dimensions, please consult the Machinery's Handbook, available for purchase at many bookstores.

Features of a Thread
A thread consists of three repeating features: a crest, flank, and root (see drawing to the right). Except in special cases, threads have symmetrical sides inclined at equal angles when a vertical line is drawn through the center of a crest or root. The distance between corresponding points on adjacent threads is known as the pitch of the thread. The flank angle is defined as the angle the flank makes with a perpendicular ray drawn from the screw axis. Unless otherwise stated, threads have a flank angle of 30°, resulting in a total angle between flanks of 60°. Each feature is shown in the diagram to the right.

The major diameter is taken from the crests of a thread while the minor diameter is taken from the roots. For most screws, crests and roots do not terminate at a sharp point, so crest and root truncation values are included in the definitions of major and minor diameter. The pitch diameter is approximately halfway between the major and minor diameters.

Thread Form
A thread form is a set of rules that define the features' scale relative to one another. Common thread forms include the Unified Screw Thread Form, used in the United States of America and measured in imperial units, and the ISO Metric Screw Thread Form, used in many parts of the world and measured with the International System of Units. There are many thread forms in the Unified screw thread standard designated by either UN, which defines a flat root contour, or UNR, which defines a round root contour. These can be further described by appending more letters. For example, an extremely fine thread with a flat root contour is designated UNEF. Those forms which are not standardized by the Unified screw thread system are designated UNS.

Thread Series
Most screws are identified by their thread series. Thread series are denoted by the major diameter and density of threads. Unified threads specify density in threads per inch, while Metric threads specify the thread pitch. For example, in the Unified nomenclature, a 1/4"-20 cap screw has a 1/4" diameter barrel and the pitch is 20 threads per inch (TPI). In metric nomenclature, an M4 x 0.7 cap screw has a 4 mm barrel and the pitch is 1 thread per 0.7 mm. The term M4 x 0.7 is often shortened to just M4.

Unified Thread Class Tolerancing
Location Loose Optimal Strict
Internal 1B 2B 3B
External 1A 2A 3A
Metric Thread Tolerance Positions
Location Loose Optimal Strict
Internal - G H
External e or f g h
Metric Thread Tolerance Grades
Dimension Location Tolerance Gradesa
Minor Diameter Internal 4, 5, 6, 7, 8
Major Diameter External 4, 6, 8
Pitch Diameter Internal 4, 5, 6, 7, 8
External 3, 4, 5, 6, 7, 8, 9
  • The tolerance becomes looser as the grade increases. The underlined grades are used with normal lengths of thread engagement.

Thread Class
The tolerances and allowances on a thread series are given by a thread class. Unified thread classes are alphanumeric identifiers starting with a number from 1 through 3, where 1 is the loosest tolerance and 3 is the tightest, and either A for external threading or B for internal threading.

Metric threads have a slightly more complex tolerancing method that uses tolerancing grades, designated by a number 3 through 9; and tolerancing positions, which use letters e through h. Grades provide a measure of the tolerance itself: the smaller the number, the tighter the tolerance. Positions denote the distance of the tolerance from the pitch diameter. Uppercase positioning letters indicate internal threads while lowercase positioning letters indicate external threads.

Quoting from the Machinery's Handbook, 29th Edition, p. 1885: "To designate the tolerance class, the grade and position of the pitch diameter is shown first followed by that for the major diameter in the case of the external thread or that for the minor diameter in the case of the internal thread, thus 4g6g for an external thread and 5H6H for an internal thread. If the two grades and positions are identical, it is not necessary to repeat the symbols, thus 4g, alone, stands for 4g4g and 5H, alone, stands for 5H5H."

Thorlabs' SM Series Threads
Threading specifications for our SM threads, utilized in our lens tube and cage system components, are given below so that you can machine mating components to suit your application. Most SM series threads utilize a non-standard Unified thread form, indicated by the letters UNS, with a 30° flank angle and a thread class of 2A and 2B. The exception is our SM30 series thread, which is a Metric thread form with a 30° flank angle and a tolerance of 6H/6g. We also offer products with C-Mount and RMS threads, and the specifications for these threads are given below for reference. Please note that other manufacturers may have different tolerances for C-Mount and RMS threads. For other thread specifications that are not listed here, please contact Tech Support.

SM05 Threading: Ø1/2" Lens Tubes, 16 mm Cage Systems
External Thread, 0.535"-40.0 UNS-2A Internal Thread, 0.535"-40.0 UNS-2B
Max Major Diameter 0.5340" Min Major Diameter 0.5350"
Min Major Diameter 0.5289" Min Pitch Diameter 0.5188"
Max Pitch Diameter 0.5178" Max Pitch Diameter 0.5230"
Min Pitch Diameter 0.5146" Min Minor Diameter (and 83.3% of Thread) 0.508"
Max Minor Diameter 0.5069" Max Minor Diameter (and 64.9% of Thread) 0.514"
RMS Threading: Objective, Scan, and Tube Lenses
External Thread, 0.800"-36.0 UNS-2A Internal Thread, 0.800"-36.0 UNS-2B
Max Major Diameter 0.7989" Min Major Diameter 0.8000"
Min Major Diameter 0.7934" Min Pitch Diameter 0.7820"
Max Pitch Diameter 0.7809" Max Pitch Diameter 0.7866"
Min Pitch Diameter 0.7774" Min Minor Diameter (and 83.3% of Thread) 0.770"
Max Minor Diameter 0.7688" Max Minor Diameter (and 64.9% of Thread) 0.777"
C-Mount Threading: Machine Vision Lenses, CCD/CMOS Cameras
External Thread, 1.000"-32.0 UN-2A Internal Thread, 1.000"-32.0 UN-2B
Max Major Diameter 0.9989" Min Major Diameter 1.0000"
Min Major Diameter 0.9929" Min Pitch Diameter 0.9797"
Max Pitch Diameter 0.9786" Max Pitch Diameter 0.9846"
Min Pitch Diameter 0.9748" Min Minor Diameter (and 83.3% of Thread) 0.966"
Max Minor Diameter 0.9651" Max Minor Diameter (and 64.9% of Thread) 0.974"
SM1 Threading: Ø1" Lens Tubes, 30 mm Cage Systems
External Thread, 1.035"-40.0 UNS-2A Internal Thread, 1.035"-40.0 UNS-2B
Max Major Diameter 1.0339" Min Major Diameter 1.0350"
Min Major Diameter 1.0288" Min Pitch Diameter 1.0188"
Max Pitch Diameter 1.0177" Max Pitch Diameter 1.0234"
Min Pitch Diameter 1.0142" Min Minor Diameter (and 83.3% of Thread) 1.008"
Max Minor Diameter 1.0068" Max Minor Diameter (and 64.9% of Thread) 1.014"
SM30 Threading: Ø30 mm Lens Tubes
External Thread, M30.5 x 0.5 – 6H/6g Internal Thread, M30.5 x 0.5 – 6H/6g
Max Major Diameter 30.480 mm Min Major Diameter 30.500 mm
Min Major Diameter 30.371 mm Min Pitch Diameter 30.175 mm
Max Pitch Diameter 30.155 mm Max Pitch Diameter 30.302 mm
Min Pitch Diameter 30.059 mm Min Minor Diameter (and 83.3% of Thread) 29.959 mm
Max Minor Diameter 29.938 mm Max Minor Diameter (and 64.9% of Thread) 30.094 mm
SM1.5 Threading: Ø1.5" Lens Tubes
External Thread, 1.535"-40 UNS-2A Internal Thread, 1.535"-40 UNS-2B
Max Major Diameter 1.5339" Min Major Diameter 1.535"
Min Major Diameter 1.5288" Min Pitch Diameter 1.5188"
Max Pitch Diameter 1.5177" Max Pitch Diameter 1.5236"
Min Pitch Diameter 1.5140" Min Minor Diameter (and 83.3% of Thread) 1.508"
Max Minor Diameter 1.5068" Max Minor Diameter (and 64.9% of Thread) 1.514"
SM2 Threading: Ø2" Lens Tubes, 60 mm Cage Systems
External Thread, 2.035"-40.0 UNS-2A Internal Thread, 2.035"-40.0 UNS-2B
Max Major Diameter 2.0338" Min Major Diameter 2.0350"
Min Major Diameter 2.0287" Min Pitch Diameter 2.0188"
Max Pitch Diameter 2.0176" Max Pitch Diameter 2.0239"
Min Pitch Diameter 2.0137" Min Minor Diameter (and 83.3% of Thread) 2.008"
Max Minor Diameter 2.0067" Max Minor Diameter (and 64.9% of Thread) 2.014"
SM3 Threading: Ø3" Lens Tubes
External Thread, 3.035"-40.0 UNS-2A Internal Thread, 3.035"-40.0 UNS-2B
Max Major Diameter 3.0337" Min Major Diameter 3.0350"
Min Major Diameter 3.0286" Min Pitch Diameter 3.0188"
Max Pitch Diameter 3.0175" Max Pitch Diameter 3.0242"
Min Pitch Diameter 3.0133" Min Minor Diameter (and 83.3% of Thread) 3.008"
Max Minor Diameter 3.0066" Max Minor Diameter (and 64.9% of Thread) 3.014"
SM4 Threading: Ø4" Lens Tubes
External Thread, 4.035"-40 UNS-2A Internal Thread, 4.035"-40.0 UNS-2B
Max Major Diameter 4.0337" Min Major Diameter 4.0350"
Min Major Diameter 4.0286" Min Pitch Diameter 4.0188"
Max Pitch Diameter 4.0175" Max Pitch Diameter 4.0245"
Min Pitch Diameter 4.0131" Min Minor Diameter (and 83.3% of Thread) 4.008"
Max Minor Diameter 4.0066" Max Minor Diameter (and 64.9% of Thread) 4.014"

Posted Comments:
lars.kreilkamp  (posted 2019-01-04 10:43:42.343)
Can you provide data on the difference in optical density for p-polarized and s-polarized light when the filters are used under an angle (e.g. at 45° or 22.5°)? Thank you, Lars
nbayconich  (posted 2019-02-28 03:32:21.0)
Thank you for contacting Thorlabs. We will be adding additional plots to our website in the near future regarding the S and P polarization states of these filters. In the meantime we can provide scan services for individual optics upon request. I will reach out to you directly to discuss our scanning capabilities.
mliphardt  (posted 2017-11-20 15:34:10.43)
Your product page contains a graph that shows: Transmission and Optical Density of Reflective UVFS ND Filters. Could you send the data in tabulated form to me? Thank you, Martin
nbayconich  (posted 2017-12-26 10:48:29.0)
Thank you for contacting Thorlabs. I will reach out to you directly with the raw data.
Andreas.Brand  (posted 2013-11-21 14:42:54.197)
Hi, you should give the filters a fixed angle of some degrees so that any reflections are deviated off axis and won't produce ghost images close to the actual image when used with a camera i.e.! kind regards Andreas Brand
pbui  (posted 2013-11-21 12:09:44.0)
Response from Phong at Thorlabs: Thank you for your feedback. I will discuss your comments with our optics department. We are constantly looking for ways to improve our products and will consider your suggestion of adding the wedge to our ND filters to eliminate ghost reflections for imaging applications.
tcohen  (posted 2013-01-02 11:00:00.0)
Response from Tim at Thorlabs: We do not yet have tested data on the NDUV. However, the performance of our ND filter line, assuming a 5mm 532nm beam, could handle your 10W/cm^2.
pantoine  (posted 2012-12-20 11:17:14.013)
What is the damage threshold for the UV reflective ND filters with CW laser operating at 532nm? The 0.75W/cm2 in the specifications looks pretty low? Typically my laser intensity exceeds slightly 10W/cm2? Thank you for your feedback
Tyler  (posted 2008-10-03 09:22:54.0)
A response from Tyler at Thorlabs: The presentation has been reworked and some performance plots added. Thank you for taking the time to point what additional information was needed to make a decision on the suitability of our product for your application.
thorlabs  (posted 2008-09-08 18:51:40.0)
Wideband or not? The overview states 1. "designed to be used in the 200-400 nm", and 2. "flat spectral response from the VIS to the NIR". If #2 is correct, there is no reason not to use this in the visible and infrared. A plot of OD vs wavelength would be a great addition. Poor wording: "A metallic Inconel coating is used to attenuate the light in these filters because of its durability and flat spectral response from the VIS to the NIR. However, Inconel will oxidize if used in an environment where the temperature is in excess of 100 °C." The "however" does not make sense here, since the two sentences refer to completely different properties of the filter.
Tyler  (posted 2008-06-30 13:52:04.0)
A response from Tyler at Thorlabs to tom.burton: Because this product can be used over an extremely wide wavelength range it would be difficult to provide a comprehensive damage threshold specification. As a result, the number provided is conservatively low. If you end up using this product, consider posting the power density and wavelength information from your application so that others in the scientific community may benefit from your experience. This forum is intended to be used as a way for people at Thorlabs to communicate with our customers and as a way for our customers to share their experiences with each other. An engineer will be in contact with you to see if we can offer further guidance.
tom.burton  (posted 2008-06-23 17:29:30.0)
cant imagine .75W/cm^2 is right on UV REFLECTIVE ND FILTERS.
Back to Top

Ø1/2" UV Fused Silica Metallic ND Filters, Mounted

Item # Optical Densitya
(Transmission)
Transmission Data
NDUV509A 0.9 (13%) info
NDUV510A 1.0 (10%) info
NDUV513A 1.3 (5%) info
NDUV515A 1.5 (3%) info
NDUV520A 2.0 (1%) info
NDUV530A 3.0 (0.1%) info
NDUV540A 4.0 (0.01%) info
  • The optical density for each filter is specified at 300 nm and with a tolerance of ±5%. The transmission is nominal. Click on More Info Icon for a plot and downloadable data.
Item # Optical Densitya
(Transmission)
Transmission Data
NDUV501A 0.1 (79%) info
NDUV502A 0.2 (63%) info
NDUV503A 0.3 (50%) info
NDUV504A 0.4 (40%) info
NDUV505A 0.5 (32%) info
NDUV506A 0.6 (25%) info
NDUV507A 0.7 (20%) info
NDUV508A 0.8 (16%) info
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
NDUV501A Support Documentation
NDUV501ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.1
$55.83
Today
NDUV502A Support Documentation
NDUV502ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.2
$55.83
Today
NDUV503A Support Documentation
NDUV503ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.3
$55.83
Today
NDUV504A Support Documentation
NDUV504ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.4
$55.83
Today
NDUV505A Support Documentation
NDUV505ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.5
$55.83
Today
NDUV506A Support Documentation
NDUV506ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.6
$55.83
Today
NDUV507A Support Documentation
NDUV507ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.7
$55.83
Today
NDUV508A Support Documentation
NDUV508ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.8
$55.83
Today
NDUV509A Support Documentation
NDUV509ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 0.9
$55.83
Today
NDUV510A Support Documentation
NDUV510ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 1.0
$55.83
Today
NDUV513A Support Documentation
NDUV513ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 1.3
$55.83
Today
NDUV515A Support Documentation
NDUV515ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 1.5
$55.83
Today
NDUV520A Support Documentation
NDUV520ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 2.0
$68.01
Today
NDUV530A Support Documentation
NDUV530ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 3.0
$68.01
Today
NDUV540A Support Documentation
NDUV540ASM05-Threaded Mount, Ø1/2" UVFS Reflective ND Filter, OD: 4.0
$68.01
Today
Back to Top

Ø25 mm UV Fused Silica Metallic ND Filters, Mounted

Item # Optical Densitya
(Transmission)
Transmission Data
NDUV09A 0.9 (13%) info
NDUV10A 1.0 (10%) info
NDUV13A 1.3 (5%) info
NDUV15A 1.5 (3%) info
NDUV20A 2.0 (1%) info
NDUV30A 3.0 (0.1%) info
NDUV40A 4.0 (0.01%) info
  • The optical density for each filter is specified at 300 nm and with a tolerance of ±5%. The transmission is nominal. Click on More Info Icon for a plot and downloadable data.
Item # Optical Densitya
(Transmission)
Transmission Data
NDUV01A 0.1 (79%) info
NDUV02A 0.2 (63%) info
NDUV03A 0.3 (50%) info
NDUV04A 0.4 (40%) info
NDUV05A 0.5 (32%) info
NDUV06A 0.6 (25%) info
NDUV07A 0.7 (20%) info
NDUV08A 0.8 (16%) info
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
NDUV01A Support Documentation
NDUV01ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.1
$70.69
Today
NDUV02A Support Documentation
NDUV02ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.2
$70.69
Today
NDUV03A Support Documentation
NDUV03ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.3
$70.69
Today
NDUV04A Support Documentation
NDUV04ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.4
$70.69
Today
NDUV05A Support Documentation
NDUV05ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.5
$70.69
Today
NDUV06A Support Documentation
NDUV06ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.6
$70.69
Today
NDUV07A Support Documentation
NDUV07ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.7
$70.69
Today
NDUV08A Support Documentation
NDUV08ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.8
$70.69
Today
NDUV09A Support Documentation
NDUV09ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 0.9
$70.69
Today
NDUV10A Support Documentation
NDUV10ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 1.0
$70.69
Today
NDUV13A Support Documentation
NDUV13ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 1.3
$77.53
Today
NDUV15A Support Documentation
NDUV15ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 1.5
$77.53
Today
NDUV20A Support Documentation
NDUV20ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 2.0
$84.05
Today
NDUV30A Support Documentation
NDUV30ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 3.0
$84.05
Today
NDUV40A Support Documentation
NDUV40ASM1-Threaded Mount, Ø25 mm UVFS Reflective ND Filter, OD: 4.0
$84.05
Today
Back to Top

Ø50 mm UV Fused Silica Metallic ND Filters, Mounted

Item # Optical Densitya
(Transmission)
Transmission Data
NDUV2R09A 0.9 (13%) info
NDUV2R10A 1.0 (10%) info
NDUV2R13A 1.3 (5%) info
NDUV2R15A 1.5 (3%) info
NDUV2R20A 2.0 (1%) info
NDUV2R30A 3.0 (0.1%) info
NDUV2R40A 4.0 (0.01%) info
  • The optical density for each filter is specified at 300 nm and with a tolerance of ±5%. The transmission is nominal. Click on More Info Icon for a plot and downloadable data.
Item # Optical Densitya
(Transmission)
Transmission Data
NDUV2R01A 0.1 (79%) info
NDUV2R02A 0.2 (63%) info
NDUV2R03A 0.3 (50%) info
NDUV2R04A 0.4 (40%) info
NDUV2R05A 0.5 (32%) info
NDUV2R06A 0.6 (25%) info
NDUV2R07A 0.7 (20%) info
NDUV2R08A 0.8 (16%) info
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
NDUV2R01A Support Documentation
NDUV2R01ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.1
$113.44
Today
NDUV2R02A Support Documentation
NDUV2R02ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.2
$113.44
Today
NDUV2R03A Support Documentation
NDUV2R03ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.3
$113.44
Today
NDUV2R04A Support Documentation
NDUV2R04ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.4
$113.44
Today
NDUV2R05A Support Documentation
NDUV2R05ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.5
$113.44
Today
NDUV2R06A Support Documentation
NDUV2R06ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.6
$113.44
Today
NDUV2R07A Support Documentation
NDUV2R07ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.7
$113.44
Today
NDUV2R08A Support Documentation
NDUV2R08ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.8
$113.44
Today
NDUV2R09A Support Documentation
NDUV2R09ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 0.9
$113.44
Today
NDUV2R10A Support Documentation
NDUV2R10ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 1.0
$113.44
Today
NDUV2R13A Support Documentation
NDUV2R13ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 1.3
$122.58
Today
NDUV2R15A Support Documentation
NDUV2R15ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 1.5
$122.58
Today
NDUV2R20A Support Documentation
NDUV2R20ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 2.0
$131.31
Today
NDUV2R30A Support Documentation
NDUV2R30ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 3.0
$131.31
Today
NDUV2R40A Support Documentation
NDUV2R40ASM2-Threaded Mount, Ø50 mm UVFS Reflective ND Filter, OD: 4.0
$131.31
Today
Back to Top

Ø25 mm UV Fused Silica Metallic ND Filter Kit

  • Nickel Coated Ø25 mm SM1-Mounted Filters
  • Optimized for UV Operation Down to 200 nm
  • UVFS Reflective ND Filter With OD 1.3 Also Available (NDUV13A, Sold Separately)


Item# Size Mount Included Storage Box Included Optical Densities
NUK01 Ø25 mm SM1 KT01 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 1.0, 2.0, 3.0, 4.0
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
NUK01 Support Documentation
NUK01Box with 10 UVFS Reflective Ø25 mm ND Filters, SM1-Mounted, 200 - 1200 nm
$767.41
Today
Back to Top

Storage Box for Mounted Filters

  • Designed to Hold Ø25 mm and Ø50 mm Mounted ND Filters
  • Protects Optics from Dust and Scratches
  • Foam Inserts Separate Optics

The KT01 and KT02 are designed to hold filters that are housed in SM1-and SM2-threaded mounts respectively.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
KT01 Support Documentation
KT01Storage Box for Mounted Ø1" (25 mm) Round Optics (Max. Capacity: 10)
$105.13
Today
KT06 Support Documentation
KT06Storage Box for Mounted Ø2" Round Optics (Max. Capacity: 10)
$105.13
Today